(1) Field of the Invention:
The present invention is directed to an acid soluble copper metal salt intensifier for use in a treatment fluid for a subterranean well, the treatment fluid being introduced within a high alloy steel member.
(2) Discussion of the Prior Art:
In the life of a subterranean oil or gas well, it frequently occurs that the production zone within the well must be chemically treated or "stimulated" to enhance the economical production life of the well. It many instances, it is common practice to introduce into the well for contact with or injection into the production zone a highly acidic solution, generally having a pH from between about 1 and 6.9. Because of the acidic nature of such a treatment fluid, the production (or workover) conduit which is utilized in the well in such applications can be expected to encounter considerable acidic corrosion which, in turn, can cause surface pitting, embrittlement, loss of metal component, and the like.
In earlier years of producing subterranean wells, the vast majority of production and workover conduits which were utilized either temporarily or permanently in the well and through which a treatment or stimulation fluid was introduced into the well comprised carbon steels, such as J-55, P-105, N-80, and the like. Recently, however, due primarily to the drilling and completion of many subterranean wells through formations which contain hydrogen sulfide, carbon dioxide, brine, and combinations of these constituents, the production and workover conduits for use in the wells have been made of high alloy steels. The high alloy steels, as used herein, include stainless steels, high nickel content steels, and steels containing alloy 625 or C-276 in clad plates, or the like.
Stainless steels, first commercially developed in the 1920's, obtain their corrosion resistance by incorporation of a surface oxide film or adsorbed oxygen, of about 10 to 100 angstroms thickness. These stainless steels may be classified by their general structure and properties as: (1) martensitic; (2) ferritic; (3) austenitic; (4) duplex; and (5) precipitation-hardening steels.
Martensitic alloy steels are magnetic and are hardenable by heat treating procedures. In subterranean well environments, they may be used for mild corrosion and high temperature service. Typical of such martensitic alloys is UNS S41000 (alloy 410) which contains from between about 11.5% and about 13.5% chromium, about 0.15% carbon and no nickel.
Ferritic alloys are similar to martensitic alloys in that they, also, are magnetic. However, ferritic alloys ar not hardenable by heat treatment and have corrosion resistance between alloys 410 and 304. They are also immune to chloride stress corrosion cracking and have a ductile to brittle transition temperature which somewhat limits their use in subterranean oil well environments. Exemplary of such ferritic alloys is UNS S44735, which contains from between about 28.0 to about 30.0% chrome, about 1% nickel, from between about 3.6% to about 4% molybdenum, and trace amounts of copper, nitrogen, titanium and niobium.
Austenitic stainless steels are non-magnetic and hardenable by cold work, and, like ferritic alloys, are not hardenable by heat treatment. Typical of such stainless steels is UNS S31603 (Alloy 316L), which contains from between about 6 and 18% chrome, from between about 10 and about 14% nickel, with traces of copper and molybdenum. Also typical of such austenitic stainless steels is UNS N08020 (Alloy 20); UNS N08825 (Alloy 825); and UNS N08904 (Alloy 904L), which contains from between about 19 and about 23% chrome, from between about 23 and about 45% nickel, and from between about 2 and about 5% molybdenum, with small percentages of copper along with other elements. Variants of these steels, such as S31254, N08026 and N08925, which contain up to about 6% molybdenum, are also classified as austenitic stainless steels and have high chloride resistance, and are particularly effective when exposed and utilized in such environments.
Duplex steels combine ferrite and austenite steels and have 2 to 3 times a yield strength of the austenitic stainless steels. A duplex stainless steel family is resistant to pitting and crevice corrosion and has significantly better CSCC resistance, than do the 300 series stainless steel products. Such steels have favorable toughness and ductility properties, with a coefficient of expansion nearer to that of carbon steel, thus reducing stress factors. Heat transfer in such stainless steels is about 25% greater than that of the austenitic steels.
Precipitation-hardening stainless steels attribute their high strength to the precipitation of a constituent from a super saturated solid solution through a relatively simple heat treatment but do not encounter a loss in resistance to corrosion or ductility. These steels may be heat treated. Typical of such steels are UNS S17400 (17-4 PH) and UNS S15700 (PH 15-7 Mo), which contains from between about 14 to about 16% chromium, and from 2 to 3% molybdenum, with from between about 6.5% and about 7.8% nickel.
Other high alloy steels include those having high nickel content. Typical of such high nickel alloys are UNS N10276 (Alloy C-276); UNS N06625 (Alloy 625); and UNS N06110. These high nickel alloy materials are used to prepare tubular goods for subterranean wells, and other components for use within subterranean wells where such use is expected to encounter extremely corrosive environments. The high nickel alloys have high tolerance to extremely hostile environments and typically contain about 60% nickel, from between about 15 to about 20% chromium, and from between about 9 and about 16% molybdenum.
U.S. Pat. No. 3,773,465 is typical of the prior art with respect to treatment of low alloy, or N-80-type production conduits with intensified acid corrosion inhibitor compositions, and discloses the treatment of such conduits with cuprous iodide.
In the present invention, it has been found that high alloy steels, as opposed to low alloy members, may be effectively protected against the effects of acid corrosion by utilizing an acid soluble copper metal salt intensifier.